1. Field of the Invention
The present invention relates to a detector and, more particularly, to a detector for an automatic gain control amplifier circuit (referred to as an "AGC amplifier" hereinafter).
2. Description of the Prior Art
An AGC amplifier includes an amplifier for amplifying an input signal and a detector circuit for generating a gain control voltage in response to the amplitude of the output signal from the amplifier. The gain control voltage is fed back to the amplifier to control the gain thereof.
The detector includes generally a rectifier circuit rectifying the output signal of the amplifier to output a rectified signal, a smoothing circuit smoothing the rectified output to output a smoothed signal, and a control circuit generating the gain control voltage by comparing the smoothed signal with a control voltage.
Such a detector is shown in FIG. 1, in which true and complementary signals are represented by signal sources 213 and 214, respectively. These signals are supplied to a full-wave rectifier circuit 2 along with a bias voltage V.sub.B. The rectifier 2 consists of transistors 201 and 202 and a constant current source 3 and produces a full-wave rectified signal at the common emitter of transistors 201 and 203. This signal is smoothed by a smoothing circuit 4 consisting of a resistor 204 and a capacitor 211. A d.c. voltage corresponding to the amplitude of the output signal of the amplifier is thus obtained. This d.c. voltage is compared with a control voltage 259 by a control circuit 5 consisting of transistors 250 and 251, resistors 253 to 256 and a constant current source 252, so that the gain control voltage is produced between output terminals 360 and 361.
Assuming that the voltages of the input signals 213 and 214 are V.sub.IN1 and V.sub.IN2, respectively; the collector currents of the transistors 201 and 202 are I.sub.C1 and I.sub.C2, respectively; and the common emitter voltage of the transistors 201 and 202 is V.sub.E, the following equations (1) and (2) are obtained: ##EQU1## where v.sub.T is the thermal voltage of the transistor and I.sub.S is the reverse saturation current of the transistor.
Since the input voltages V.sub.IN1 and V.sub.IN2 are represented as follows: EQU V.sub.IN1 =v.sub.in +V.sub.B, (3) EQU V.sub.IN2 =-v.sub.in +v.sub.B, (4)
where v.sub.in is the a.c. component of the signals 213 and 214, the equations (1) and (2) change as follows: ##EQU2## From equations (5) and (6), equation (7) is obtained: ##EQU3## Assuming that the mutual conductance of the rectifier circuit 2 is gm and the current of the current source 3 is 2I.sub.O (I.sub.O &gt;gm v.sub.in), the following equations (8) and (9) are derived: EQU I.sub.C1 .apprxeq.I.sub.O +2v.sub.in gm, (8) EQU I.sub.C2 .apprxeq.I.sub.O -2v.sub.in gm. (9)
Accordingly, Eq. (7) can be rewritten as follows: ##EQU4## With the above-mentioned assumption of I.sub.O &gt;v.sub.in gm Eq. (11) becomes as follows: ##EQU5## Further, using the expression for gm in terms of v.sub.T and I.sub.O ##EQU6## Accordingly, V.sub.E can be taken out from the common emitter of the transistors 201 and 202, and can be given by ##EQU7## Next, if the signal is assumed to be a sine wave with amplitude V.sub.a, namely, v.sub.in =V.sub.a sin .omega.t, where .omega. is the angular frequency of the signal, V.sub.E given by Eq. (14) can be given by the following equation: ##EQU8## Therefore, the detected amplitude output after smoothing V.sub.Ea is given by: ##EQU9## The output after the smoothing is taken out from a terminal 7, and is a dc voltage which is directly proportional to the square of the signal amplitude Va.sup.2 and is inversely proportional to the thermal voltage v.sub.T.
Next, the smoothed output V.sub.Ea is supplied to the control circuit 5 and compared with a variable voltage source 259 as a control voltage. If the voltage of the variable voltage source 259 is V.sub.X1, the current of the current source 252 is 2I.sub.O, and the resistances of the resistors 253, 254, 255 and 256 are set to be R.sub.L, R.sub.L, R.sub.E and R.sub.E, respectively, the differential output V.sub.DET1 (the differential voltage between the terminals 360 and 361) of the control circuit 5 is given by ##EQU10##
This voltage V.sub.DET1 is used for the gain control of the amplifier.
The overall construction of the AGC amplifier including the detector is shown in FIG. 2. More specifically, an AGC detector 304 is constructed as shown in FIG. 1, and the gain control voltage from the detector 304 is amplified by a differential amplifier 301 and then supplied to a variable gain amplifier 350 as a gain control voltage V.sub.AGC. The true and complementary outputs of the amplifier 350 are supplied to an emitter-follower amplifier 310, the outputs of which are connected to the output terminals 312 and 313 and further to the detector 304 through a differential amplifier 303. The output terminals 312 and 313 are connected to the following circuit which is not shown. The variable gain amplifier 350 is of a double differential amplifier type which is widely used in an IC circuit. An input signal v.sub.1A to be amplified with a gain control is supplied to the base of a transistor 325, and the outputs of the amplifier 350 are derived from the collectors of transistors 321 and 324. The gain of the variable gain amplifier 350 is controlled by the difference in voltage between the common base of the transistors 321 and 324 and that of the transistors 322 and 323. When the voltage difference becomes smaller than 0.1 V, the gain of the amplifier 350 starts to change. In other words, the AGC function starts to operate when the differential voltage becomes 0.1 V. Since the voltage difference is amplified by the differential amplifier 301, the AGC function starts to operate when the difference in input voltage to the differential amplifier 301 is 1 mV, assuming the gain of the differential amplifier 301 to be 100. Thus, the AGC starts to operate when the gain control voltage V.sub.DET1 of the control circuit 5 satisfies the following condition: EQU V.sub.DET1 .apprxeq.0. (18)
Namely, from Eqs. (18) and (17), when EQU V.sub.Ea .perspectiveto.V.sub.X1, (19)
The AGC starts to operate. From Eqs. (16) and (19) there is obtained ##EQU11##
Referring to FIG. 3, the voltage source 259 (FIG. 1) includes an operational amplifier 401, resistors R.sub.x and Ry, a transistors 402 and a current source 404. Therefore, the voltage V.sub.X1 at the terminal 260 can be given as follows: ##EQU12## From Eqs. (20) and (21) there is obtained ##EQU13## Since Va is the amplitude of the input signal v.sub.in of the AGC detector in the AGC amplifier shown in FIG. 2, the output of the AGC amplifier is smaller than the signal v.sub.in by a factor of the gain of the post-stage amplifier 303. Assuming that the output of the AGC amplifier is v.sub.oa and the gain of the post-stage amplifier 303 is A.sub.op, the signal v.sub.in is represented as follows: EQU v.sub.in =A.sub.op v.sub.oa. (23)
Accordingly, the following equation is obtained: EQU V.sub.a =A.sub.op V.sub.oa, (24)
where V.sub.oa is the amplitude of v.sub.oa. The substitution of Eq. (24) into Eq. (22) results in as follows: ##EQU14## The above equation shows that V.sub.oa which is the output amplitude of the AGC amplifier can be controlled by the resistance ratio of R.sub.y to R.sub.x. That is, the output of the AGC amplifier can be controlled by the variable voltage source 259.
However, as apparent from equation (26), the output amplitude V.sub.oa of the AGC amplifier depends heavily on the terminal voltage v.sub.T of the transistor. For example, V.sub.oa changes by about 17% for a temperature change of 100.degree. C. so that the output of the AGC amplifier is scattered over a wide range depending upon the temperature. Moreover, V.sub.X1 is given by the circuit shown in FIG. 3, resulting in increase in the number of elements.